North Korea has borders with both China and Russia (at Khasan). The Korean peninsula is separated from mainland China by the Korea Bay (in the north) and the Yellow Sea.
(see map link)
Hong Kong is an island, and is completely surrounded by water, separating it from the People's Republic of China.
The Sea of Japan separates the island of Japan from mainland Korea.
sea of Japan
high rugged mountains
Korea
The Sea of Japan
Japan,South Korea and North Korea
-- "The Yellow Sea"In response to the previous answer, the Yellow Sea separates The Korean Peninsula and Mainland China.The 'Sea of Japan' separates Japan from the Asian mainland (including the Korean Peninsula). The 'Korean Strait' also separates the southern coast of South Korea from Japan.Please see a labelled map in the related links below.
Japan is separated from mainland East Asia by the Korean Strait and Sea of Japan.
There is the Gulf of Suez that separates the Sinai Peninsula (in Asia) from the Egyptian mainland (Africa).
It is the Korea Strait that separates Korea and Japan.
Japan is considered to be part of Asia. Japan is separated from the Asian mainland by the Sea of Japan, coming closest at the Korea Strait.
The Pacific Ocean separates Asia and California, The Arctic Ocean separates Alaska from northern Asia.